This invention relates to battery chargers and, in particular, to chargers offering a fast tapered charge for encapsulated nickel cadmium batteries. However, other types of storage battery may be charged by means of the present invention.
A great deal has been written on the subject of charging nickel cadmium batteries. It is well known, for example, that the peculiarities of these batteries require careful management if the full lifespan of the batteries is to be realized.
Prior art battery chargers have provided either trickle charging at low currents, fast charging using high currents or a combination of both. Trickle charging is usually too slow as it takes typically 12 to 14 hours to charge a fully discharged battery. On the other hand, fast charging includes the risk of damaging the battery or reducing its life due to overheating. Nevertheless, fast charging is often preferred as it possible with care to charge a battery in less than one hour. In fast charging, an appropriate parameter of the battery (e.g., output voltage or battery temperature) is usually monitored to detect when it is fully charged. Chargers combining fast and trickle charging allow automatic battery charging in which the battery is quickly charged and then kept topped up by a trickle charge.
It is an object of the present invention to provide a method and apparatus for charging batteries which overcomes, or least ameliorates, the disadvantages of the prior art.
Accordingly, the invention provides a method of charging batteries including the following steps:
applying to the battery a first current of sufficient amplitude to provide a fast charge while the output voltage of the battery does not exceed a first predetermined reference voltage;
terminating the first current a first predetermined interval after the output voltage exceeds the first predetermined reference voltage;
allowing the battery to self-discharge while the output voltage exceeds the first predetermined reference voltage, defining thereby a self-discharge interval; and
repeating the preceding steps while the self-discharge interval is less than a first predetermined multiple of the first predetermined interval.
Preferably, the method of the invention further includes the steps of:
producing a measure of battery temperature; and
reducing the first predetermined reference voltage in proportion to an increase in the measure of battery temperature.
Preferably, the method of the invention includes the initial steps of:
applying to the battery, for a predetermined test interval, a test current of sufficient amplitude to provide a fast charge;
comparing the output voltage of the battery with a predetermined acceptance reference voltage;
first control means responsive to the first comparator for inhibiting the first charging means a predetermined interval after the output voltage of the battery exceeds the first predetermined reference voltage, the first control means thereby allowing the battery to self-discharge while the output voltage exceeds the first predetermined reference voltage defining a self-discharge interval;
second control means for inhibiting the first charging means while the self-discharge interval is less than a first predetermined multiple of the first predetermined interval and producing an indication of completion of charging.
Preferably the apparatus of the invention includes temperature sensing means for producing a measure of the battery temperature and means for reducing the first predetermined reference voltage in proportion to an increase in the measure of battery temperature.
Preferably, also, the apparatus includes second charging means for charging the battery for a second predetermined interval with a second current of sufficient amplitude to provide a fast charge;
a second comparator for comparing the output voltage of the battery with a predetermined acceptance reference voltage; and
third control means for inhibiting the first and second charging means if the output voltage does not exceed the second predetermined acceptance reference voltage and to provide an indication of rejection of the battery.
The apparatus preferably includes means for applying a load to the battery when the self-discharge interval exceeds a second predetermined multiple of the first predetermined interval, said second predetermined multiple being less than the first predetermined multiple, said load being insufficient to substantially discharge the battery;
signal means for producing a signal indicative of the difference between the output voltage of the battery under the application of said load with the output voltage of the battery under the application of no load; and
fourth control means responsive to the difference signal for inhibiting the first charging means if the difference signal exceeds a predetermined amount and to provide an indication of rejection of the battery.
An apparatus according to the invention will preferably include third charging means for charging the battery with a third current of sufficient amplitude to provide a trickle charge.